Final Fantasy X Walkthrough
Air Ship - Sin - The Mouth

At least you can save before this point! You'd blasted Sin into the ground. But he's up again of course. You go up onto the top deck of the ship to talk to Yuna, and she's worried about how the Fayth will help. And then the attack comes.

Now you're going to fight with Sin's mouth, with this strange time-bar. You have LIMITED TIME. Every round his mouth opens a little more, and if it opens fully you die. This means your strategy is a bit different. Using aeons is generally bad, because they get ONE attack, and then he opens his mouth. At least with your 3 people, you get 3 attacks which usually add up to more than a single aeon attack.

Sin has 140,000hp. So you have to do some MASSIVE attacks. Since you just healed up if you have an uber-aeon attack ready (i.e. over 15,000hp) it might be worth it to do that, but in general it's probably best to concentrate on the very top attacks you have. Often this is Wakka doing a blast, Yuna doing a Holy, and then whichever third person you have as the most powerful at the moment. Be sure to have HASTE cast on everybody so they go as quickly as possible, giving the most attacks possibly in between Sin's mouth-opening.